1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to an illumination apparatus and method for a display device, and more particularly, to an illumination apparatus and method for a display device constructed such that an incident ray impinges upon a hologram or hologram micro-pattern at an angle which provides high diffraction efficiency.
2. Description of the Related Art
Non-emissive displays such as liquid crystal displays (LCDs) typically require a separate light source such as an LCD backlight. The LCD backlight needs to provide a uniform illumination.
FIG. 1 schematically shows a conventional LCD backlight using a point light source. Referring to FIG. 1, a plurality of light emitting diodes (LEDs) 41 are arranged in parallel along one side of a light guide plate (LGP) 43. Light rays emitted by the plurality of LEDs 41 are incident on a microscopic hologram pattern 45 formed on the top surface of the LGP 43 as they propagate in the LGP 43. The light rays incident on the hologram pattern 43 are diffracted by the hologram pattern 45, are emitted vertically through the top surface of the LGP 43, and are incident on an LCD panel (not shown).
The hologram pattern 45 causes incident light to emit onto the LCD panel and is oriented in a predetermined direction. When light is incident upon the hologram pattern 45 from a specific azimuth angle and altitude angle, the incident light can be diffracted and exit with highest efficiency. FIG. 2 is a diagram for explaining the azimuth angle and altitude angle of a ray incident on the hologram pattern 45. As shown in FIG. 2, the azimuth angle is an angle of an incident ray with respect to an axis perpendicular to a direction of the hologram pattern 45, and the altitude angle is an angle of an incident ray with respect to an axis perpendicular to a surface of the hologram pattern 45. To achieve highest diffraction efficiency, the azimuth angle is 0° (i.e., incident beam is normal to the holographic diffraction pattern 45) and the altitude angle is usually near 50° as shown in FIG. 3.
Since the LED 41 has a radiation angle of about 45°, there are dead zones on which light is not incident at a portion of the hologram pattern 45 closest to a space between the LEDs 41. On the other hand, incident rays emitted by the LED 41 overlap each other so the intensity of light increases at a portion of the hologram pattern 45 that is distant from the LEDs 41. Here, a light radiation angle refers to an angle at which light intensity becomes half the maximum light intensity. FIG. 4 is a graph of light intensity vs. LED radiation angle. Referring to FIG. 4, as the radiation angle increases, the light intensity decreases. The light intensity decreases by half when the radiation angle is 45°. Thus, an angle at which light radiated from the plurality of LEDs 41 is incident on the hologram pattern 45 varies depending on the location of the hologram pattern 45. Consequently, an emission angle and light intensity vary across the hologram pattern 45.
Referring to FIG. 1, to solve this problem, the conventional LCD backlight includes an element for collimating an incident ray at an incident portion of the LGP 43. However, the conventional LCD backlight provides low diffraction efficiency because a highest-intensity incident ray is incident on the hologram pattern 45 at an altitude angle of 90°. That is, the highest-intensity ray is a ray radiated from the center of a LED (see FIG. 4), which propagates parallel to the LGP 43 within the LGP 43. Thus, as evident from FIG. 3, the highest-intensity ray is incident on the hologram pattern 45 at 90° for which diffraction efficiency is approximately 0. Conversely, only a ray with light intensity that is half the maximum intensity is diffracted from the hologram pattern 45 with the highest efficiency. As a result, since only 80% of the rays incident to the LGP 43 exits from the top surface of the LGP 43, the efficiency of light utilization is low.